Hydrocarbon drilling operations utilize information relating to parameters and conditions downhole during drilling. Such information may comprise characteristics of the earth formations surrounding the borehole, along with data relating to the size and direction of the borehole itself. The collection of information relating to conditions downhole is termed “logging.”
In early hydrocarbon prospecting, drilling operations and logging operations where separate and distinct operations. Logging a well required removing or “tripping” the drilling assembly to insert a wireline logging tool to collect the data. As drilling technology advanced, aspects of logging tools became part of the drill string, and specifically the bottom hole assembly (BHA), such that data could be collected contemporaneously with the drilling processing.
Systems for measuring conditions downhole, such as the movement and position of the drilling assembly, have come to be known as “measuring-while-drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, have come to be known as “logging-while-drilling” techniques, or “LWD”. The terms MWD and LWD often are used interchangeably. For purpose of this disclosure, the term MWD will be used with the understanding that this term may encompass both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
In MWD systems, sensors in the drill string measure drilling parameters and in some cases formation characteristics. While drilling is in progress, data from these sensors is continuously or intermittently transmitted to a surface detector by some form of telemetry. Most MWD systems use the drilling fluid (or mud) in the drill string as the information carrier, and are thus referred to as mud-pulse telemetry systems. In positive-pulse systems, a valve or other form of flow restrictor creates pressure pulses in the fluid flow by adjusting the size of a constriction in the drill string (e.g., positive-pressure system). In negative-pulse systems, a valve creates pressure pulses by releasing fluid from the interior of the drill string to the annulus, bypassing the drilling bit (e.g., negative-pulse systems). In both system types, the pressure pulses propagate at the speed of sound through the drilling fluid to the surface, where they are detected by various types of transducers.
Some related art positive-pulse systems create the positive pulse by actuating a pilot valve, and the pilot valve in turn actuates a main poppet valve to cause a temporary flow restriction and/or blockage and thus an increased pressure pulse. Such systems have reliability issues in that particles in the drilling fluid tend to accumulate in and around the pilot valve, which degrades performance of the pilot valve. Eventually the particle accumulation in and around the pilot valve disables the pilot valve, and thus disables the ability to create pulses.